1. Field of the Invention
The present invention relates to a micro electro mechanical system (MEMS) switch, and more particularly to a MEMS switch having a piezoelectric drive system using a piezoelectric body thin film, and to a method of manufacturing same.
2. Description of the Related Art
In recent years, there has been extensive research and development of micro electro mechanism systems (MEMS) applied to semiconductor manufacturing processes, and switch structures of various types have been proposed in the field of MEMS switches, which is one example of the application of MEMS technology (see, for example, International Publication No. WO 2004/019362 A1 and Japanese Patent Application Publication No. 2007-157511).
International Publication No. WO 2004/019362 A1 proposes a structure in which a plurality of actuator fingers having alternately arranged positive and negative electrodes are provided as a drive source on a surface of a movable membrane, and on/off switching operation of a switch is produced by causing the membrane to deform by means of the electrostatic force of attraction between these electrodes. Furthermore, International Publication No. WO 2004/019362 A1 proposes a composition in which a group of electrodes is also disposed on the rear surface side of the membrane in order to perform compulsory driving in a switching off operation, and also proposes a composition in which a piezoelectric material is filled in between the electrodes instead of an air gap therebetween.
Japanese Patent Application Publication No. 2007-157511 discloses a structure in which a voltage is applied between a first switch drive electrode disposed on a substrate and a second switch drive electrode provided on a diaphragm which is disposed on the substrate via a cavity (gap), and the diaphragm is caused to bend by the electrostatic force of attraction generated between the electrodes, whereby a charge accumulating electrode and the first switch drive electrode make contact with each other via a tunnel insulating film.
However, MEMS switches using piezoelectric methods which have been proposed in the related art entail the following:
(1) Decline in High-Speed Response Due to Accumulation of Drive Charge and Dielectric Loss (Sticking Phenomenon)
In the case of a switching operation by attraction or repulsion due to electrostatic force, it becomes impossible to follow high-speed on/off driving in a high-frequency range, due to accumulation of charge in the piezoelectric bodies which are adjacent to the electrodes. Further, there is a possibility of not being able to follow high-speed on/off driving in a high-frequency range due to dielectric loss of the adjacent piezoelectric bodies in the high-frequency range.
(2) Time-Dependent Sticking Phenomenon of Electrostatically Attracted Contacts
In the case of a switching operation by means of attraction and repulsion due to electrostatic force, in order to guarantee a reliable on/off switching operation, it is necessary to increase the attraction distance (air gap) and raise the drive voltage during attraction, as far as possible. In this case, a sticking phenomenon is liable to occur in the attracted contact portion. The “sticking phenomenon” referred to here is phenomenon of adherence which is not intended and which causes the contact to become stuck permanently.
(3) Power Consumption of Drive Circuit, Cost of Circuit, and Size of Circuit
In the case of miniature devices such as mobile devices in particular, the circuitry of the drive system must be kept simple, but with a related-art electrostatic drive system, it has been necessary to raise the drive voltage and therefore power consumption has become large. Moreover, the drive circuit in a case of a driving operation by attraction and repulsion based on electrostatic force is required to correspond to both positive and negative voltage drivings, and the extent to which costs and circuit size can be reduced is limited because a drive circuit such as a drive IC has to be compatible with positive and negative voltages.
In response to the above, the present inventor has investigated MEMS switches using piezoelectric drive methods and has discovered technical issues of the following kind.
[1] Switch Contact Faults Due to Warping when Forming a Thin Film on a Membrane Diaphragm
When a piezoelectric thin film is formed directly on a membrane, then although it depends on the size of the membrane, there is a possibility that the membrane diaphragm portion is warped by the stress in the piezoelectric film. If the amount of warping is uniform, then warping can be corrected by means of the set drive voltage, and the like, but if the amount of warping is not uniform, then it can lead to contact faults in the switch contacts, and reliable on/off operation cannot be guaranteed.
[2] Reliability in Switching Off Operation
If a piezoelectric film is formed on one side of a membrane and the membrane is caused to bend by applying a voltage to turn the switch on, whereas the drive voltage to the piezoelectric body is halted to turn the switch off, the switching off operation being performed on the basis of the spring force of the membrane (a force which seeks to return the membrane to its original state), then it is difficult to guarantee reliable separation of the contact. Moreover, there is also a possibility that high-speed response becomes poor.
In the case of a piezoelectric film, it is also possible to perform a driving in the opposite direction by applying an opposite voltage, but in general, driving in the opposite direction cannot be performed unless the applied voltage is equal to or less than the coercive voltage of the piezoelectric body. Supposing that a voltage equal to or greater than the coercive voltage is applied, then reversal of the polarization of the piezoelectric body occurs and the body is actually driven so as to be displaced in the forward direction again. Consequently, there is little margin for generating a large force in the opposite direction by applying a large voltage in order to guarantee reliable separation of the contact.
[3] Increased Costs of Drive Control System by Opposite Potential Driving for the Purpose of Forced Separation when Switching Off
Furthermore, in order to apply a potential in the opposite direction, it is necessary to design a drive circuit to be compatible with positive and negative voltages, and therefore the costs and size of the drive circuit, such as a drive IC, are increased.